Electron Capture in Collisions of Slow Highly Charged Ions with an Atom and a Molecule: Processes and Fragmentation Dynamics

International audienceProcesses involved in slow collisions between highly charged ions (HCI) and neutral targets are presented. First, the mechanisms responsible for double electron capture are discussed. We show that, while the electron-nucleus interaction is expected to be dominant at projectile velocities of about 0.5 a.u., the electron-electron interaction plays a decisive role during the collision and gains importance when the projectile velocity decreases. This interaction has also to be invoked in the capture of core electrons by HCI. Finally, the molecular fragmentation of H2 following the impact of HCI is studied

Dynamique de la fragmentation de molécules simples induite par impacte d'ion multichargé

The aim of this work is to study the dynamics of swift multicharged ion-induced fragmentation of diatomic (CO) and triatomic (CO2) molecules. Performed at the GANIL facility, this study used the Recoil Ion Momentum Spectroscopy technique (RIMS), which consists of time-of-flight mass spectrometer, coupled with a multi-hit capability position sensitive detector (delay line anode). The high-resolution measurement of the kinetic energy distribution released (KER) during the CO fragmentation points out the limitation of the Coulomb Explosion Model, revealing, for example, the dication CO2+ electronic state contribution in the case of C+/O+ fragmentation pathway. Furthermore, the multi-ionization cross section dependence with the orientation of the internuclear axis of CO is compared with a geometrical model calculation. Finally, different behaviours are observed for the dissociation dynamics of a triatomic molecule (CO2). While triple ionization leads mainly to a synchronous concerted fragmentation dynamics, a weak fraction of dissociating molecule follows a sequential dynamics involving CO2+ metastable states. In the case of double ionization, (CO2)2+ dication dissociation dynamics is asynchronous concerted and has been interpreted using a simple model involving an asymmetrical vibration of the molecule.Ce manuscrit présente une étude de la dynamique de la fragmentation de molécules diatomiques (CO) et triatomiques (CO2) induite par impact d'ions multichargés rapides, dans un régime de vitesse de collision où l'ionisation de la cible est le processus dominant. Réalisée au GANIL, cette étude a nécessité la mise en œuvre une technique dite de "Spectroscopie d'Impulsion d'Ions de Recul" consistant en l'association d'un spectromètre de masse par temps de vol et d'un détecteur à localisation multi-impact basé sur une anode à ligne à retard. La mesure à haute résolution des distributions d'énergie cinétique (KER) des fragments issus de la dissociation de la molécule CO a permis de séparer et d'identifier les états électroniques excités du dication (CO)2+ peuplés lors de la collision, confirmant ainsi les limitations du modèle de l'explosion coulombienne. Le rôle de l'orientation de l'axe intermoléculaire de la molécule CO par rapport à la direction du faisceau projectile incident sur les sections efficaces a été mesuré et comparé aux prédictions d'un modèle géométrique. Enfin, différents comportements de la dynamique plus complexe de la fragmentation de la molécule CO2 ont été mis en évidence. Notamment, la triple ionisation de CO2 conduit majoritairement à une fragmentation concertée synchrone. Néanmoins, un comportement minoritaire a pu être observé et identifié comme associé à une fragmentation séquentielle impliquant un état métastable de l'ion moléculaire (CO)2+. Par contre, la fragmentation de l'ion moléculaire (CO2)2+ présente un caractère de type concerté asynchrone interprété à l'aide d'un modèle simple, mettant en jeu un mode de vibration asymétrique de la molécule

Caractérisation de faisceaux de protons produits par laser via une analyse optique et nucléaire de films RCF

Generation of particle beams with high intensity lasers opens new domains of investigation both for fundamental and applied research. These beams are often characterized by means of radiochromic films (RCF) 1,2,3 which provide information both on their energy and spatial distributions. However the optical response function of these films for protons depends on their precise composition and is not well known making it difficult to extract absolute information on the number of particles contained in the beam. Proton induced nuclear reactions in RCF produce radioactive elements. The analysis of these activities allows to determine in a completely independent way the energy distribution of the proton beams 4,5. With the Monte Carlo simulation code Geant4, we have established the nuclear activation response function for different stacks of RCF films or RCF films and copper foils. We will present our method making use of the optical and nuclear activation responses of RCF and discuss the results obtained in the analysis of proton beams generated with the LULI 100 TW laser

Characterization of laser induced proton beams with radiochromic films: comparison between optical densitometry and nuclear activation analyses.

The generation of particle beams with high intensity lasers opens new domains of investigation both for fundamental and applied research. These beams are often characterised with radiochromic films (RCF) which provide information both on their energy and spatial distributions. However the optical response function of these films for protons depends on their precise composition and is not well known making it difficult to extract absolute information on the number of particles in the beam. Proton induced nuclear reactions in RCF produce radioactive elements. The analysis of these activities allows to determine in a completely independent way the energy distribution of the proton beams. Using the Monte Carlo simulation code Geant4, we have established the nuclear activation response functions for different stacks of RCF films and stacks of RCF films and copper foils. We will present our method based on the optical and nuclear activation responses of RCF and discuss the results obtained in the analysis of proton beams generated with the LULI 100 TW laser